This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-290801, filed on Sep. 25, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a high-output amplifier using a transistor amplifier.
2. Background of the Invention
In the field of wireless communication systems, digital communication such as CDMA (code division multiple access) is currently the major system, there is a demand for a high-output amplifier (1) having a high gain, (2) having a high efficiency, (3) less influenced by temperature changes, and (4) linearly operative in output levels of a wide dynamic range. In addition to those, in systems like portable telephones using low source voltages, such a high-output amplifier is additionally required to be (5) operative with a low control voltage.
As a conventional high-output amplifier, there is a method using an emitter-grounded bipolar transistor for class AB operation to supply a bias current/voltage by means of a base bias circuit. The principle and characteristics of this method are explained below.
To obtain a high-output amplifier with a high gain, an emitter-grounded bipolar transistor is used as the transistor amplifier of the high-output amplifier. The high-output amplifier is made up of a control circuit and the transistor amplifier, and a bipolar transistor is typically used as the transistor amplifier. For connection of the bipolar transistor, although there are emitter-grounded circuits, base-grounded circuits, collector-grounded circuits (emitter follower circuits), and so on, because of a high gain, emitter-grounded circuits are used in an overwhelmingly majority of amplifier circuits. Emitter-grounded circuits are configured to use the emitter as a common terminal for both the input and the output, apply an input signal to the base and extract the output from the collector.
In order to obtain a high-efficiency high-output amplifier, the bias condition of the above-mentioned emitter-grounded transistor amplifier is adjusted for class AB to perform class AB operation. That is, a high-efficiency amplifier will be obtained by adjusting the bias condition for class B (in which power consumption is essentially zero when the input is zero) instead of class A (in which a considerable amount of current is always supplied to the transistor). Actually, however, since class B operation will increase the distortion by fluctuation of the gain because of the non-linearity of the mutual conductance of devices, by a way of use as class AB (in which a small d.c. current=idling current flows into the transistor even when the input is zero), a high-efficiency high-output amplifier can be obtained.
Next, for the purpose of obtaining a high-output amplifier less affected by temperature fluctuation, a bias is supplied to the control terminal (base) of the transistor amplifier through a bias circuit. That is, in high-output amplifiers using bipolar transistors, major circuits are those using the base/emitter voltage as the reference. However, temperature coefficient of the output current is fairly large, a base bias circuit is used as one of control circuits to prevent the base voltage/current from largely changing with temperature. Such high-output amplifiers include a first conventional high-output amplifier using a bias circuit shown in FIG. 13. The bias circuit of FIG. 13 includes a bipolar transistor Q1 and a resistor R1, and uses a diode-connection current mirror circuit made by short-circuiting the base and the collector to supply a bias voltage and a bias current Ib.
As reviewed above, by using a class AB emitter-grounded bipolar transistor, a high-output amplifier having (1) a high gain and (2) a high efficiency can be obtained, and by using the method of supplying the bias current/voltage by using the first conventional bias circuit shown in FIG. 13, a high-output amplifier (3) less affected by temperature fluctuations. Further, since the bias circuit of FIG. 13 uses a single stage transistor, (5) the control voltage may be low.
The first conventional high-output amplifier using the bias circuit of FIG. 13, however, cannot satisfies the requirement of (5) linear operation in output levels over a wide dynamic range. That is, in emitter-grounded bipolar transistor for class AB operation, since the average collector current increases in response to the output level, the bias circuit has to supply a sufficient additional amount corresponding to the increase of the average base current responsively; however, the bias circuit as shown in FIG. 13 cannot supply a sufficient amount of current because a voltage drop by the resistor R1 occurs.
A second conventional high-output amplifier using a bias circuit of FIG. 14 linearly operates in output levels over a wide dynamic range. The circuit of FIG. 14 is a current mirror circuit that supplies the base current through the emitter follower, etc., in which Q is a bipolar transistor, R is a resistor, Vcc is the source voltage, and a current can be supplied by decreasing the output impedance.
The second conventional high-output amplifier using the bias circuit of FIG. 14, however, involves the problem that (5) the control voltage of the bias circuit inevitably increases. That is, in the bias circuit of FIG. 14, using two-stage transistors, the bias current Ib fluctuates largely with temperature unless the control voltage Vcont is much higher than twice as large as the ON voltage of the transistors. For example, in the circuit of FIG. 14, let the operation voltage of the bipolar transistor Q be 1.2V, then the control voltage Vcont must be much higher than 2.4V. So, if a control voltage around 2.7V is used for operation, the bias current will be seriously affected by temperature changes. If, however, the control voltage is raised, it invites a serious problem in systems like portable telephones that use low source voltages.
As discussed above, with conventional emitter-grounded transistor amplifiers biased for class AB, although it has been possible to obtain a high-output amplifier having (1) a high gain and (2) a high efficiency, but because of insufficient characteristics of base bias circuits of transistor amplifiers, it has been difficult to obtain a high-output amplifier (3) less affected by temperature changes, (4) linearly operative in output levels over a wide dynamic range and (5) controllable with a low control voltage.
According to an embodiment of the invention, there is provided a high-output amplifier comprising:
a bias circuit (BC) having a first bias circuit and a second bias circuit, said first bias circuit including a first transistor (Q1) having one end connected to a first high-voltage-side source (Vcont) and the other end connected to a low-voltage-side source (Vss), said one end being connected to the control terminal of the first transistor, said second bias circuit including a second transistor (Q2) having one end connected to a second high-voltage-side source (Vcc), the other end connected to said one end of said first transistor (Q1), and a control terminal connected to said first high-voltage-side source (Vcont); and
a transistor amplifier (RF1) having a control terminal connected to said one end of said first transistor (Q1), one end as an output terminal of said high-output amplifier, and the other end connected to a low-voltage-side source.